Method and device for determining the remaining serviceable life of a product

ABSTRACT

A method and a device for acquiring performance quantities of a product, in particular until its technical failure, and for determining the remaining service life of the product are described. To permit the most accurate possible estimate of life not based on a model for any products having or accessing a performance data memory without storage of signal curves over time, the determination of the remaining service life of the product, acquisition of service lives of the products and determination of service life threshold values are performed on the basis of performance quantities subdivided into classes (classified). Weighting factors are determined first and then these weighting factors are used to determine weighted, cumulative service lives and service life threshold values. The reliability of s=1 . . . S products is monitored in mass production.

BACKGROUND INFORMATION

[0001] The present invention relates to a method and a device for determining the remaining service life of a product; the present invention also relates to a method and a device for acquiring the service life until technical failure of the product as well as methods and a device for determining service life threshold values of products as a function of certain time-variable performance quantities for monitoring the reliability of products, and finally the present invention also relates to a device arranged in a product whose reliability is to be monitored, this device being used to compare the actual service life of the product with service life threshold values according to the preambles of the independent claims.

[0002] German Patent Application 195 16 481 A1 describes a method of determining a life. A control device for a motor vehicle is described, having a performance data memory in which performance quantities of the vehicle are stored, these quantities being capable of providing information regarding the probability of failure and/or the future reliability of the control device. Essential data on the life history of a control device is stored in the performance data memory to permit a conclusion to be drawn with regard to the reliability of the control device as needed.

OBJECT AND ADVANTAGES OF THE PRESENT INVENTION

[0003] The object of the present invention is to permit the most accurate possible estimate (not based on a model) of service life of any desired products having or accessing a performance data memory. Another object of the present invention is to achieve optimum acquisition of data and storage in a performance data memory to permit optimum utilization of the memory, in particular to save on memory.

[0004] To achieve this object, starting with a method of acquiring service lives until technical failure of a product, the present invention proposes that values of certain performance quantities be acquired, the value range of the individual performance quantities be subdivided into classes, and the service life be acquired as a function of the class in which the acquired value of the performance quantity falls.

[0005] In addition, the present invention proposes for achieving this object a method and a device for determining the remaining service life of a product until technical failure, values of a value range of at least one performance quantity of the product being acquired, the value range of the performance quantity being subdivided into classes and a service life of the product being determined for each class and stored in a performance data memory assigned to the product, preselectable weighting factors being assigned to the service lives and thus at least one weighted cumulative service life being determined for the product, the weighted cumulative service life being compared with at least one preselectable service life threshold value and the remaining service life of the product being determined on this basis.

[0006] The product whose service life until technical failure is acquired is designed, for example, as a control device or a subsystem (e.g., brakes, engine, transmission, steering, etc.) of a motor vehicle, for example. The products have a performance data memory and/or are assigned to such a memory, where the acquired performance quantities, i.e., the service lives, are stored and may be called up again as needed. The performance data memory preferably has a nonvolatile memory (e.g., an EEPROM or a flash EEPROM) as well as means for acquiring the performance quantities, i.e., the service lives. In the case of a motor vehicle, the performance data memory may be implemented in one or more control devices, for example.

[0007] Discrete system states (e.g., the number of starting operations, the number of emergency starts, the number of thermal shutdowns, etc.) as well as the time-variable performance quantities are acquired with the performance data memories. For example, sensor data such as temperature, current, voltage, pressure, etc. are acquired as performance quantities.

[0008] The value range is subdivided into a plurality of classes linearly or nonlinearly in the allowed value range of performance quantities under operating conditions. Extreme values that would result in immediate destruction of the product are outside the allowed value range. The class assignment is based on the classification of the entire value range in relevant load groups. The individual classes have different effects on the aging/wear of the product. The service life of the product for each performance quantity in each class is acquired in the performance data memory.

[0009] According to the present invention, the individual technical service life of a product is determined and the degree of wear at a given point in time is calculated on the basis of performance quantities subdivided into classes (so-called classified performance quantities). On the basis of the classified performance quantities, an especially reliable and accurate determination of the service life of a product is possible, the memory demand for the performance data memory being minimized because it is possible to refrain from acquiring time characteristics of the performance quantities. This permits particularly reliable preventive maintenance/repairs just before reaching the end of the technical service life.

[0010] According to a preferred refinement of the present invention, it is proposed that the values of the performance quantities be acquired at regular intervals in time and that a class counter of a certain class be incremented if the value of an acquired performance quantity falls in this class. Thus, after acquisition of the service lives, a service life histogram may be assigned to each performance quantity of a certain product, this histogram indicating the service life of the product for the performance quantity within a certain class. The size in bytes of the performance data memory required for storage of the performance data is obtained from the multiplication product of:

[0011] the number of performance quantities,

[0012] the average number of classes per performance quantity, and

[0013] the average number of bytes per class counter.

[0014] The method according to the present invention for acquiring service lives on the basis of classified performance quantities has special advantages in particular in determination of service life threshold values of products for monitoring the reliability of products. Therefore, according to an advantageous refinement of the present invention, a method of determining service life threshold values of the type defined above is proposed, wherein

[0015] the service lives of the products until technical failure of the product are determined for the classes of the performance quantities by using the method according to claim 1 or 2;

[0016] weighting factors are assigned to the classes of the performance quantities;

[0017] the weighting factors are determined by solving an optimization problem

min {f(x)}, where x={a_ij, t_ijk}

[0018]  taking into account the correlation between the individual performance quantities;

[0019] cumulative service lives for the individual performance quantities that are critical for the products are determined from the equation ${{P\_ iz}{\_ crit}} = {\underset{j = 1}{\overset{M\_ i}{SUM}}\left\{ {{a\_ ij} \times {t\_ ijz}} \right\}}$

[0020]  and

[0021] for the individual products, the service life threshold values are determined from the equation min {P_iz_crit},  where  i = 1    …  N  or ${\frac{1}{N} \times \underset{i = 1}{\overset{N}{SUM}}\left\{ {{P\_ iz}{\_ crit}} \right\}},\quad {{{where}\quad i} = {1\quad \ldots \quad N}}$

[0022] The individual classes have different effects on the aging/wear of the products. Therefore, weighting factors which express the relative influence of a certain class of a certain performance quantity on the aging and/or wear of the product are assigned to the classes of performance quantities. The present invention proposes that the weighting factors be determined from a subset K of the products and this then be applied to subset Z of the products. In this way, the critical weighted cumulative service lives of the performance quantities for serial use may be determined for the products from subset S such that on reaching these service lives an end to the technical service life may be deduced.

[0023] The weighting factors are determined by solving an optimization problem

min{f(X)}, where x={a_ij, t_ijk}

[0024] taking into account the correlation between the individual performance quantities, where a_ij is the weighting factor assigned to class j of performance quantity i, and t_ijk is the service life of product k for class j of performance quantity i. The correlation between the performance quantities may be taken into account, for example, by the fact that the weighting factors are determined from an equation system in which the weighted cumulative service lives for each performance quantity are linked together by operators. The operators may be, for example, an AND link (forming a product), an OR link (forming a sum) or a fuzzy link (e.g., an intermediate state between AND and OR).

[0025] The critical cumulative service lives for the individual performance quantities which, when reached, permit the inference that the product in question is at the end of its technical service life are to be defined after the weighting factors have been determined by solving an optimization problem using suitable mathematical optimization algorithms. To do so, with the help of K products, a number of Z products are operated until technical failure, the weighting factors calculated from the K products being applied to the classified performance quantities of the Z products. The following equation is determined for all performance quantities and all Z products ${{P\_ iz}{\_ crit}} = {\underset{j = 1}{\overset{M\_ i}{SUM}}\left\{ {{a\_ ij} \times {t\_ ijz}} \right\}}$

[0026] where P_iz_crit denotes the critical cumulative service life of product z of performance quantity i and t_ijz is the service life of product z for class j of performance quantity i. This yields Z vectors of the weighted cumulative service lives as follows

Y_z={P_(—)1z_crit, P_(—)2z_crit, . . . , P_Nz_crit},

[0027] where z=1 . . . Z.

[0028] The service life threshold values which, when reached, indicate that the product will soon be at the end of its technical life are determined for the individual products from the column minimums of matrix Y_z according to the equation

min{P_iz_crit}, where i=1 . . . N

[0029] or from the average of the column elements of matrix Y_z according to the equation ${\frac{1}{N} \times \underset{i = 1}{\overset{N}{SUM}}\left\{ {{P\_ iz}{\_ crit}} \right\}},\quad {{{where}\quad i} = {1\quad \ldots \quad {N.}}}$

[0030] This functions with the required reliability if the individual column elements are close enough together, i.e., if the standard deviation of the column elements is not too great. Freak values should not be taken into account in selecting the column minimums.

[0031] After the critical cumulative service lives for the individual performance quantities have been determined, the need for a repair, replacement or maintenance may be signaled by the product shortly before reaching the critical threshold value in the case of all mass-produced products equipped with performance data memories. As an alternative, the performance quantities stored in the product may be analyzed as part of a regular product maintenance program.

[0032] In summary, k=1 . . . K products are first operated until technical failure in order to be able to determine weighting factors a_ij. Then, weighting factors a_ij are integrated into the performance data memory of z=1 . . . Z products which are operated again until technical failure in order to determine the critical cumulative service lives P_iz_crit and to determine the service life threshold values by way of a minimum selection or the average of critical cumulative service lives P_iz_crit. Accordingly, the reliability of s=1 . . . S products is monitored in serial use, the actual service life of a product s being compared with a threshold value.

[0033] According to a preferred embodiment of the present invention, it is proposed that the weighting factors be determined by solving the optimization problem $\underset{i = 1}{\min\limits^{N}}\left\{ {\underset{k = 1}{\overset{K}{SUM}}\quad \underset{j = 1}{\overset{M\_ i}{SUM}}\quad {ABS}\left\{ {{{SUM}\left\{ {{a\_ ij} \times {t\_ ijk}} \right\}} - 1} \right\}} \right\}$

[0034] with the inequality secondary condition a_ij>0, where a_ij is the weighting factor assigned to class j of performance quantity i and t_ijk is the service life of product k for class j of performance quantity i. According to this embodiment, no correlation between the individual performance quantities is taken into account in calculation of the weighting factors. It is thus assumed that each performance quantity may result in technical destruction of the product regardless of the values of the other performance quantities.

[0035] If no correlation between the individual performance quantities is assumed for determination of the weighting factors, the largest ratio of a weighted cumulative service life for a performance quantity to the critical threshold value of the performance quantity may be interpreted as the degree of wear. The remaining residual life in % is then calculated according to the equation

Remaining life [%]=1−Degree of wear [%].

[0036] According to an alternative embodiment of the present invention, it is proposed that the weighting factors be determined by solving the optimization problem $\overset{K}{\min\limits_{v = 1}}\left\{ \overset{\quad}{\left. {{\overset{K}{\underset{\mu = 1}{SUM}\quad}\quad \underset{\mu \neq v}{\overset{N}{SUM}}\quad \underset{i = 1}{\overset{M\_ i}{\quad {ABS}}}\left\{ {\underset{j = 1}{{PROD}\left\{ {SUM} \right.}\quad \left\{ {{a\_ ij} \times {t\_ ij}\quad \overset{N}{\mu}} \right\}} \right\}} - {\underset{i = 1}{\overset{M\_ i}{PROD}}\left\{ {\underset{j = 1}{SUM}\quad \left\{ {{a\_ ij} \times {t\_ ijv}} \right\}} \right\}}} \right\} \quad} \right.$

[0037] with the inequality secondary condition a_ij>0. In this embodiment, the correlation between the individual performance quantities is taken into account. It is thus assumed that a plurality of performance quantities together result in technical destruction of the product. According to this embodiment, the performance quantities are linked together by pure AND links (forming a product). The weighting factors are determined so that the weighted class sums of each product linked by the AND operator are a minimum “distance” from one another.

[0038] In a third alternative embodiment, a plurality of performance quantities are linked at the level of individual classes. It is assumed here that a plurality of performance quantities within certain classes result in technical destruction of the product.

[0039] To achieve the object of the present invention, it is additionally proposed, starting from a device for acquiring the service lives until technical failure of a product, that the device have first means for acquiring the values of certain performance quantities at regular intervals in time, the value range of the individual performance quantities being subdivided into classes and the device having second means for acquisition of the service lives as a function of the class in which the acquired value of the performance quantity falls.

[0040] According to an advantageous refinement of the present invention, it is proposed that the second means shall increment a class counter of a certain class if the value of a performance quantity acquired falls in this class.

[0041] The device according to the present invention for acquiring service lives on the basis of classified performance quantities offers special advantages in particular when determining service life threshold values of products for monitoring the reliability of products. Therefore, according to an advantageous refinement of the present invention, a device for determining service life threshold values of the type defined in the preamble is proposed, wherein this device has means for carrying out the method according to one of claims 5 through 8.

[0042] To achieve the object of the present invention, starting from a device of the aforementioned type arranged in a product to be monitored, it is proposed that the service life threshold values be determined by the method according to one of claims 5 through 8. The performance data memory of the device may be particularly small because when the service life threshold values are determined according to the present invention, a memory-intensive acquisition of time characteristics of the performance quantities is unnecessary.

[0043] In addition, acquisition of performance data in classes has the advantage in particular that the memory may be utilized optimally, i.e., in particular only a small amount of memory is needed because no complicated acquisition of performance quantities over the entire time axis, i.e., with reference to the time axis, need be performed. Therefore, the present invention in particular the performance data acquisition may be implemented expediently as an additional functionality in a control device or in a device provided specifically for that purpose.

[0044] Additional advantages and advantageous embodiments are derived from the description and the features of the claims.

DRAWINGS

[0045] A preferred embodiment of the present invention is explained in greater detail below on the basis of the drawing:

[0046]FIG. 1 shows a flow chart of a method according to the present invention for acquiring service lives until technical failure of a product according to a preferred embodiment, and

[0047]FIG. 2 shows a flow chart of a method according to the present invention for determining service life threshold values of products according to a preferred embodiment.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0048]FIG. 1 shows a flow chart of a method according to the present invention for acquiring service lives t_ijk of a product k=1 . . . K until technical failure of product k according to a preferred embodiment. Product k whose service life t_ijk is acquired is designed, for example, as a control device or a subsystem (e.g., brakes, engine, transmission, steering, etc.) of a motor vehicle. Product k has a performance data memory in which acquired performance quantities i=1 . . . N and/or service lives t_ijk are stored and may be called up again as needed. The performance data memory preferably has a nonvolatile memory (e.g., an EEPROM or a flash EEPROM) as well as means for acquisition of the performance quantities and/or service lives. In the case of a motor vehicle, the performance data memory may be implemented in one or more control devices, for example.

[0049] Discrete system states (e.g., number of starting operations, number of emergency starts, number of thermal shutdowns, etc.) and time-variable performance quantities i are acquired with the performance data memories. For example, sensor data such as temperature, current, voltage, pressure and the like are acquired as performance quantities i.

[0050] The method begins in a function block 10. In a function block 11, the value range allowed under operating conditions for individual performance quantities i to be acquired is subdivided linearly or nonlinearly into classes j=1 . . . M_i. Extreme values resulting in direct destruction of product k are outside the allowed value range. The class assignment is based on the division of the entire value range into relevant load groups. Individual classes j have different effects on the aging/wear of product k.

[0051] In a downstream function block 12, values of performance quantities i are acquired at regular intervals in time. Service lives t_ijk are acquired as a function of class j in which the acquired value of performance quantity i falls. To do so, a class counter of a certain class j is incremented in a function block 13 if the value of acquired performance quantity i falls in this class j. Each performance quantity i of a certain product k may thus be assigned a service life histogram after acquisition of service lives t_ijk, this histogram yielding service life t_ijk of product k for performance quantity i within a certain class j. Service lives t_ijk are obtained from the product of the count reading of the class counter and the time interval of the acquired values of performance quantities i.

[0052] In a downstream query block 14, a check is performed to determine whether the acquisition of service lives t_ijk is concluded. If not, the operation branches off back to function block 12. If the acquisition of service lives t_ijk is concluded, the operation branches off to the end of the method in function block 15.

[0053]FIG. 2 shows a flow chart of a method according to the present invention for determining service life threshold values of products z according to a preferred embodiment. The method according to the present invention begins in a function block 20. Then service lives t_ijk of products k for class j of performance quantities i until technical failure of product k are first determined by using the method according to FIG. 1.

[0054] Then in a function block 21, weighting factors a_ij are assigned to the classes of performance quantities i. Since individual classes j have different effects on aging/wear of products k, weighting factors a_ij expressing the relative influence of a certain class j of a certain performance quantity i on the aging or wear of product k are assigned to classes j of performance quantities i.

[0055] In a downstream function block 22, weighting factors a_ij are determined by solving an optimization problem

min{f(x)}, where x={a_ij, t_ijk},

[0056] taking into account the correlation among individual performance quantities i. Weighting factors a_ij may be determined, for example, by solving the optimization problem $\underset{i = 1}{\min\limits^{N}}\left\{ {\underset{k = 1}{\overset{K}{SUM}}\quad {SUM}{\quad \quad}\underset{j = 1}{\overset{M\_ i}{ABS}}\left\{ {{{SUM}\left\{ {{a\_ ij} \times {t\_ ijk}} \right\}} - 1} \right\}} \right\}$

[0057] with inequality secondary condition a_ij>0. No correlation among individual performance quantities is taken into account, and it is assumed that each performance quantity i may result in technical failure of product k, regardless of the values of other performance quantities i.

[0058] As an alternative, weighting factors a_ij may also be determined by solving the optimization problem $\overset{K}{\min\limits_{v = 1}}\left\{ \overset{\quad}{\left. {{\overset{K}{\underset{\mu = 1}{SUM}\quad}\quad \underset{\mu \neq v}{\overset{N}{SUM}}\quad \underset{i = 1}{\overset{M\_ i}{\quad {ABS}}}\left\{ {\underset{j = 1}{{PROD}\left\{ {SUM} \right.}\quad \left\{ {{a\_ ij} \times {t\_ ij}\quad \overset{N}{\mu}} \right\}} \right\}} - {\underset{i = 1}{\overset{M\_ i}{PROD}}\left\{ {\underset{j = 1}{SUM}\quad \left\{ {{a\_ ij} \times {t\_ ijv}} \right\}} \right\}}} \right\} \quad} \right.$

[0059] with inequality secondary condition a_ij>0. The correlation among the individual performance quantities i is taken into account, and it is assumed that a plurality of performance quantities i jointly result in technical destruction of product k. Performance quantities i are linked together by pure AND links (forming a product) in this embodiment.

[0060] According to a third alternative, a linking of multiple performance quantities i at the level of individual classes j is conceivable. This is based on the assumption that multiple performance quantities i within certain classes j result in technical destruction of product k.

[0061] According to the present invention, weighting factors a_ij are determined from a subset K of products k, and these are then used for subset Z of products z. Therefore, critical cumulative service lives P_iz_crit of performance quantities i may be determined for serial use such that on reaching such a critical service life, it is possible to predict the end of the technical service life.

[0062] In a function block 23, cumulative service lives P_iz_crit for individual performance quantities i that are critical for products z may be determined from the equation: ${{P\_ iz}{\_ crit}} = {\underset{j = 1}{\overset{M\_ i}{SUM}}\left\{ {{a\_ ij} \times {t\_ ijz}} \right\}}$

[0063] by operating products z until technical failure. This yields Z vectors of the weighted cumulative service lives

Y_z=(P_(—)1z_crit, P_(—)2z_crit, . . . , P_Nz_crit),

[0064] where z=1 . . . Z.

[0065] Finally, in function block 24 the service life threshold values which, when reached, indicate that the end of the technical life of the product is imminent are determined for individual products z from the column minimums of matrix Y_z according to the equation:

min{P_iz_crit},

[0066] where i 1 . . . N

[0067] or from the average of the column elements of matrix Y_z according to the equation: ${\frac{1}{N} \times \underset{i = 1}{\overset{N}{SUM}}\left\{ {{P\_ iz}{\_ crit}} \right\}},\quad {{{where}\quad i} = {1\quad \ldots \quad N}}$

[0068] This then functions with the required reliability when the individual column elements are sufficiently close together, i.e., when the standard deviation in the column elements is small.

[0069] Freak values, if any, thus should not be taken into account in selecting the column minimums. In function block 25, the method for determining service life threshold values of products z is concluded. For determination of the service life threshold values, in addition to absolute or relative minimum selection and simple averaging, other methods and procedures such as sliding averaging or empirical averaging or harmonic averaging or formation of a meridian, etc. may also be used.

[0070] After determining critical cumulative service lives P_iz_crit for individual performance quantities i, the need for a repair, replacement or maintenance may be signaled by product s shortly before reaching the critical threshold value in the case of all mass-produced products s equipped with performance data memories. This may also take place in particular in the form of a self-diagnosis of the mass-produced product. As an alternative, the performance quantities stored in product s are analyzed as part of regular product maintenance. This product maintenance may also be performed, for example, in the case of a partial product of a vehicle or the vehicle itself in operation even in the form of onboard diagnosis.

[0071]FIG. 3 shows schematically one possible device according to the present invention, where P denotes the product itself. It is connected by a communications systems KS, in particular, a line system or a bus system to a performance data memory BSe external to the product. As an alternative, an internal performance data memory BSi may also be provided in the product itself. It is also possible for both memories to be present simultaneously and for a virtual memory of BSe and BSi to be formed, for example. The means used to implement the method according to the present invention as explained above are combined in M, e.g., in the form of a microcomputer or microcontroller. These means may also be present in a control device of a motor vehicle, for example, or may be introduced there.

[0072] Product P, whose service life is to be acquired, is designed, for example, as a control device or a subsystem (e.g., brakes, engine, transmission, steering, etc.) of a motor vehicle. Products P have a performance data memory BSi and/or they are assigned to such a memory (BSe) where the acquired performance quantities or service lives are stored and may be called up again as needed. The performance data memory preferably has a nonvolatile memory (e.g., an EEPROM or a flash memory) as well as means EM for acquisition of the performance quantities, i.e., the service lives. In the case of a motor vehicle, the performance data memory may be implemented in the form of one or more control devices, for example. Acquisition means EM acquire their information via communications system KS, for example, or other interfaces of the product, e.g., to other sensors or actuators. The analysis, the service life acquisition, service life determination by threshold value comparison, etc., are performed in particular by means M, which also initiate or perform the signaling or initiation of other measures. Acquisition means EM and means M may also be used in combination and may likewise be assigned to the performance data memories in a targeted manner, i.e., integrated into them.

[0073] Discrete system states (e.g., number of starting operations, number of emergency starts, number of thermal shutdowns, etc.) and the time-variable performance quantities are acquired with the performance data memories. For example, sensor data such as the temperature, current, voltage, pressure and the like may be acquired as performance quantities. The sensor system required for this is interfaced via communications system KS, for example, or is connected to the product by other interfaces. Depending on the product, the sensor system may also be partially or completely integrated into the product. The same is also true of actuators which supply information according to the present invention in particular.

[0074] Thus, with all mass-produced products s equipped with performance data memories, the need for repair, replacement or maintenance may be signaled by product s shortly before reaching the critical threshold value. This may also take place in particular in the form of a self-diagnosis of mass-produced product s, e.g., through performance data memory having integrated means M, i.e., acquisition means EM. 

What is claimed is:
 1. A method of determining service lives (t_ijk) of a product (k), wherein values of a value range of at least one performance quantity of the product are acquired, the value range of the performance quantity being subdivided into classes (j=1 . . . M_i), and the service life being acquired as a function of the class into which the acquired value of the performance quantity falls.
 2. A method of determining the remaining service life of a product until technical failure with determination of service lives as recited in claim 1, wherein a service life of the product is determined for each class and is stored in a performance data memory assigned to the product, preselectable weighting factors being assigned to the service lives and thus at least one weighted, cumulative service life being determined for the product, and the weighted, cumulative service life being compared with at least one preselectable service life threshold value, and the remaining service life of the product being determined therefrom.
 3. The method as recited in claim 2, wherein the determination of the remaining service life is performed in the product itself in the form of a self-diagnosis of the product, and before or when at least one service life reaches the at least one service life threshold value, this fact is signaled and suitable measures are initiated.
 4. The method as recited in claim 1 or 2, wherein the values of the performance quantities (i) are acquired at regular intervals in time, and a class counter of a certain class (j) is incremented if the value of an acquired performance quantity (i) falls into this class (j).
 5. A method of determining a service life threshold value of a product for monitoring the reliability of the product by comparing a service life with a threshold value, using a method of determining service lives as recited in claim 1, wherein the values and/or the service lives are stored according to the classes in a performance data memory assigned to the product; and a first subset of a product is operated until technical failure, so that the service lives of the classes of the preselectable performance quantities of the product are determined, a weighting factor being determined for each class and each performance quantity therefrom, this weighting factor reflecting the influence until technical failure of the product of the respective class and performance quantity, and a second subset of the product being operated until technical failure, the weighting factors determined from the first subset being applied to the second subset, and a critical service life being determined for each performance quantity over all classes in the second subset of the product, and the service life threshold value being determined from the critical service lives over all classes of all performance quantities.
 6. A method of determining service life threshold values of products (z=1 . . . Z) as a function of certain time-variable performance quantities (i=1 . . . N) for monitoring the reliability of products (s=1 . . . S), the actual service life of a product (s) being compared with a threshold value as part of monitoring, wherein the service lives (t_ijk) of the products (k) until technical failure of the product (k) are determined for the classes (j) of the performance quantities (i) by using the method according to claim 1 or 4; weighting factors (a_ij) are assigned to the classes (j) of the performance quantities (i); weighting factors (a_ij) are determined by solving an optimization problem min {f(x)}, where x={a_ij, t_ijk}  taking into account the correlation among the individual performance quantities; cumulative service lives (P_iz_crit) for the individual performance quantities (i) that are critical for the products (z) are determined from the equation: ${{P\_ iz}{\_ crit}} = {\underset{j = 1}{\overset{M\_ i}{SUM}}\left\{ {{a\_ ij} \times {t\_ ijz}} \right\}}$

 and for the individual products (z), the service life threshold values are determined from the equation: min{P_iz_crit}, where i=1 . . . N or ${\frac{1}{N} \times \underset{i = 1}{\overset{N}{SUM}}\left\{ {{P\_ iz}{\_ crit}} \right\}},{{{where}\quad i} = {1\ldots \quad N}}$


7. The method as recited in claim 1 or 5 or 6, wherein the weighting factors (a_ij) are determined by solving the optimization problem $\min \left\{ {\underset{i = 1}{\overset{N}{SUM}}\quad \underset{k = 1}{\overset{K}{SUM}}\quad {ABS}\left\{ {{\underset{j = 1}{\overset{M\_ i}{SUM}}\left\{ {{a\_ ij} \times {t\_ ijk}} \right\}} - 1} \right\}} \right\}$

 with the inequality secondary condition a_ij>0.
 8. The method as recited in claim 1 or 5 or 6, wherein the weighting factors (a_ij) are determined by solving the optimization problem $\min \left\{ {\underset{v = 1}{\overset{K}{SUM}}\quad \underset{\underset{\mu \neq V}{\mu = 1}}{\overset{K}{SUM}}\quad {ABS}\left\{ {{\overset{N}{\underset{i = 1}{PROD}}\left\{ {\underset{j = 1}{\overset{M\_ i}{SUM}}\left\{ {{a\_ ij} \times {t\_ ij}\quad \mu} \right\}} \right\}} - {\underset{i = 1}{\overset{N}{PROD}}\left\{ {\underset{j = 1}{\overset{M\_ i}{SUM}}\left\{ {{a\_ ij} \times {t\_ ijv}} \right\}} \right\}}} \right\}} \right\}$

 with the inequality secondary condition a_ij>0.
 9. A device for determining service lives (t_ijk) of a product (k), wherein first means are included which acquire the values of a value range of at least one performance quantity of the product at regular intervals in time, the value range of the performance quantity being subdivided into classes (j=1 . . . M_i), and second means being included which acquire the service life as a function of the class into which the acquired value of the performance quantity falls.
 10. A device for acquiring the remaining service life of a product until technical failure, including determination of service lives as recited in claim 9, wherein third means are included which determine a service life of the product for each class and store this data in a performance data memory assigned to the product, fourth means being included which assign preselectable weighting factors to the service lives and thus determine at least one weighted, cumulative service life for the product, and fifth means being included which compare the weighted, cumulative service life with at least one preselectable service life threshold value and determine therefrom the remaining service life of the product.
 11. The device as recited in claim 9 or 10, wherein the second means increment a class counter of a certain class (j) if the value of an acquired performance quantity (i) falls into this class (j).
 12. The device as recited in claim 9 for determining service life threshold values of products (z=1 . . . Z) as a function of certain time-variable performance quantities (i=1 . . . N) for monitoring the reliability of products (s=1 . . . S), the service life of a product (s) being compared with a threshold value as part of monitoring, wherein the device has means for performing the method as recited in one of claims 5 through
 8. 13. The device as recited in claim 9 arranged in a product (s=1 . . . S) whose reliability is to be monitored, having means for comparing the service life of the product (s) with threshold values, wherein service life threshold values according to the method as recited in one of claims 5 through 8 are used as the threshold values. 